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Cooled EGR is one of the engine technologies that has been certified by the EPA for on-highway heavy duty diesel engines to meet the EPA October 2002 2.5 g/bhp-hr NMHC + NOx and 0.1 g/bhp-hr particulate matter exhaust emission regulation. Cooled EGR as the primary exhaust emission control reduction technology also minimizes the fuel economy penalty associated with this exhaust emission regulation. The cooled EGR system however requires precise EGR rate of flow control in a very unfriendly environment that includes acidic exhaust gas condensates, static pressures up to 4 Bar, temperatures over the entire range of -40 to 250° C, and high engine vibration levels. Several technologies have been proposed and evaluated to achieve a closed loop feedback signal for the EGR flow control valve and VGT (Variable Geometry Turbocharger) vane position.

The recent availability of fast response (< 1.0 seconds) exhaust gas temperature sensors for exhaust gas temperature measurement enables a new method for closed loop feedback engine control in small engines and other non-stoichiometric applications (V-8 marine fuel injected engines for example). Conventional closed loop stoichiometric A/F control with traditional switching type oxygen sensors is often not applicable because these engines rarely if ever operate at stoichiometric A/F ratios. Richer A/F ratio control is necessary for maximum power and combustion temperature cooling. Wide range UEGO A/F sensors are an alternate solution but the cost of these sensors may be too high. Additionally, the very short exhaust systems found on many small engines can allow ambient air to back flow into the exhaust due to pressure pulsations and corrupt the signal from an oxygen concentration type sensor.

New exhaust emission laws require significant reduction of tailpipe hydocarbon emissions. The cold start phase of engine operation is a critical period when HC emissions must be minimized. High driveability index (DI) or low volatility fuel causes the open loop air/fuel (A/F) ratio during an initial cold start of the engine to shift lean which in turn contributes to unstable combustion. To compensate for this lean shift, the open loop A/F ratio must be commanded richer than necessary to allow acceptable driveability with high DI fuels which consequently increases tailpipe HC emissions. With engine cold start conditions being equal, (coolant temperature, engine speed, engine load, ignition timing, and commanded A/F ratio), the difference in cold start engine-out A/F ratio can only be attributed to the volatility characteristics of the fuel. The A/F ratio in a combustion chamber and hence, the temperature of the exhaust gas, is a function of the volatility of the fuel.

There are three primary parameters that must be controlled in an engine for optimum performance; air, fuel (A/F ratio), and ignition (spark). Similarly, there are three primary parameters that must be controlled in a three-way catalyst for optimum performance; air, fuel (A/F ratio), and energy (temperature). Currently, most engine management systems provide closed loop control on the A/F ratio via oxygen sensor feedback, but temperature control is left to open loop methods with models and calculations. This paper will describe the benefits of closed loop catalyst temperature control (directly measured) for reducing emissions and providing a more robust catalyst monitor function. This catalyst monitor function will include measurement of catalyst lightoff performance which is the dominant effect causing HC tailpipe emissions to exceed OBD threshold levels. The lightoff measurement is not effected by driving cycle.

Ultra Low Emission Vehicle (ULEV) and proposed Super Low Emission Vehicle (SULEV) requirements along with Lean Burn Systems will require improved Engine Management Systems and new catalyst technologies. High performance, reliable exhaust gas/catalyst temperature sensors are one of the key enabling technologies needed for these next generation engine management systems and catalysts. The demand for accurate, high temperature Exhaust Gas Temperature Sensors (EGTS) is growing quickly. Gasoline direct injection engine lean NOx catalyst traps, On Board Diagnostic catalyst monitoring systems, Diesel engine active lean NOx catalyst systems and large Diesel engine diagnostics and protection functions all require accurate measurement of exhaust gas temperature over a wide range. Added to these requirements are the need for short response times, low production costs, and a high sensitivity linear signal for direct input into engine management controllers.